1. Field of the Invention
The present invention relates to a phase-separation-controlled polybutadiene resin composition and a printed wiring board using the same for an insulating layer.
2. Background Art
In recent years, GHz bands have been used for signal bands of telecommunication devices such as PHS and mobile phones and for computer CPU clock time, resulting in the development of higher frequencies. Electrical signal transmission loss is expressed as the sum of dielectric loss, conductor loss, and radiation loss. The higher the electrical signal frequency, the greater the levels of dielectric loss, conductor loss, and radiation loss. Transmission loss causes electrical signal attenuation, resulting in loss of signal reliability. Thus, in the cases of wiring boards on which high-frequency signals are handled, it is necessary to devise a means of suppressing the increases in dielectric loss, conductor loss, and radiation loss. Dielectric loss is proportional to the product of the square root of the relative dielectric constant of the insulating material that is formed into a circuit, the dielectric loss tangent, and the frequency of the signal used. Therefore, it is possible to suppress the increase in dielectric loss by selecting, as an insulating material, a material having a low relative dielectric constant and a small dielectric loss tangent. As such insulating material, thermosetting polybutadiene having a structure that does not contain heteroatoms has been under examination for a long time. JP Patent Publication (Kokoku) No. 47-51952 B (1972) and JP Patent Publication (Kokoku) No. 48-14428 B (1973) each disclose a polybutadiene resin composition comprising a low-molecular-weight polybutadiene comprising 1,2-butadiene units (80 wt % or more) and having a number average molecular weight of 1000 to 20000, to which a vinyl monomer, a silane coupling agent, a filler, a curing accelerator, and a pigment are added according to need. Further, the above references each disclose a laminate produced by allowing a strengthening material such as glass fiber to be impregnated with such polybutadiene resin composition and carrying out heating and pressurization in the presence of organic peroxide. In addition, JP Patent Publication (Kokoku) No. 58-21925 B (1983) and JP Patent Publication (Kokoku) No. 58-21926 B (1983) each disclose a polybutadiene resin composition comprising, as a crosslinking component, a high-molecular-weight polybutadiene polymer comprising 1,2-butadiene units (50% or more) and having a number average molecular weight of 100000 to 200000. Such polybutadiene resin compositions were found to have the effect of imparting good tack-free properties to prepregs.